The invention relates to a new process for the sulfone from 4-aminophenol, alkali metal hydroxide and 4,4'-dihalodiphenyl sulfone in an organic solvent.
4,4'-Bis(4-aminophenoxy)diphenyl sulfone (BAPS) is an important monomer for the preparation of high-melting polyamides, polyamides and polyamidoimides. Furthermore, it is also suitable as a curing agent of epoxides.
According to DE-A-1,909,520, which is incorporated by reference herein in its entirety, BAPS is prepared from an alkali metal salt of p-aminophenol and a 4,4'-dihalodiphenylsulfone in a polar aprotic solvent, which can also be an N-alkylated acid amide, at 50.degree. to 150.degree. C. In this reaction, p-aminophenol and alkali metal hydroxide are used in equivalent amounts and the dihalo compound in a slightly less than equivalent amount, relative to the stoichiometrically required amount of p-amino-phenol. In the example, the molar ratio of p-aminophenol to 4,4'-dichlorodiphenyl sulfone is 2.063. The reaction is carried out in dimethyl sulfoxide at 100.degree. C. The product is precipitated with water, dissolved, treated with activated carbon and reprecipitated. It then has a melting point of 191.degree. to 192.degree. C., which can be increased to 193.degree. to 194.degree. C. by recrystallization.
According to DE-A-2,315,607, which is incorporated by reference herein in its entirety, BAPS is prepared from p-aminophenol, sodium hydroxide and 4,4'-dichlorodiphenyl sulfone, stoichiometric amounts of the starting compounds being used. The reaction is carried out in dimethyl sulfoxide at 160.degree. C. After two precipitations, a product having a melting point of 177.degree. to 187.degree.C. is obtained. After recrystallization, the melting point is 188.degree. to 191.degree. C.
According to Kawakami et al. (Journal of Polymer Science, Polymer Chemistry Edition, Vol. 12 (1974), 565-73), too, which is incorporated by reference herein in its entirety, BAPS is prepared from p-aminophenol, sodium hydroxide and 4,4'-dichlorodiphenyl sulfone in dimethyl sulfoxide. This article proposes a 1 to 2% molar excess of p-aminophenol, relative to sodium hydroxide. A sodium hydroxide excess is not recommended, since this can lead to the formation of nitrogen bridges. In a working example, the molar ratio of sodium hydroxide to p-aminophenol is 0.99 and that of p-aminophenol to 4,4'-dichlorodiphenyl sulfone is 2.10. This gives a product having a melting point of 189.degree. to 191.degree. C. After recrystallization, the melting point is 191.degree. to 192.degree. C.
For polycondensation purposes, very pure BAPS having a very high melting point is required. BAPS having a melting point below 191.degree. C. is still unsuitable, while a product having a melting point of 191.degree. to 193.degree. C only leads to low-quality polycondensation products having a dark color and poor mechanical properties. The known processes thus provide a BAPS which is usable for polycondensations only after several purification steps and after recrystallization and often the BAPS provided is usable only to limited degree.